Cascadia Onshore-Offshore Site Response, Submarine Sediment Mobilization, and Earthquake Recurrence

Local geologic structure and topography may modify arriving seismic waves. This inherent variation in shaking, or site response, may affect the distribution of slope failures and redistribution of submarine sediments. I used seafloor seismic data from the 2011 to 2015 Cascadia Initiative and permanent onshore seismic networks to derive estimates of site response, denoted S-n, in low- and high-frequency (0.02-1 and 1-10Hz) passbands. For three shaking metrics (peak velocity and acceleration and energy density) S-n varies similarly throughout Cascadia and changes primarily in the direction of convergence, roughly east-west. In the two passbands, S-n patterns offshore are nearly opposite and range over an order of magnitude or more across Cascadia. S-n patterns broadly may be attributed to sediment resonance and attenuation. This and an abrupt step in the east-west trend of S-n suggest that changes in topography and structure at the edge of the continental margin significantly impact shaking. These patterns also correlate with gravity lows diagnostic of marginal basins and methane plumes channeled within shelf-bounding faults. Offshore S-n exceeds that onshore in both passbands, and the steepest slopes and shelf coincide with the relatively greatest and smallest S-n estimates at low and high frequencies, respectively; these results should be considered in submarine shaking-triggered slope stability failure studies. Significant north-south S-n variations are not apparent, but sparse sampling does not permit rejection of the hypothesis that the southerly decrease in intervals between shaking-triggered turbidites and great earthquakes inferred by Goldfinger et al. (2012, 2013, 2016) and Priest et al. (2017) is due to inherently stronger shaking southward. Plain Language Summary Earthquakes radiate seismic waves that shake the surface. The shaking at a site can be altered by nearby geologic structure (the sediments and rocks beneath the surface) and topography, amplifying and prolonging or diminishing the strength of the shaking. Scientists and engineers call these effects site response and measure and account for them in the design of shaking-resilient structures. Earthquake shaking also causes failures of slopes and redistributes sediments offshore, which can break communications cables, initiate local tsunamis, alters ecosystems, and even influences how the tectonic plates move. Deposits (called turbidites) left behind by past slope failures and transient sediment-laden currents also are useful, providing a record of previous earthquakes spanning millennia. I used unique seismic data from the seafloor and from permanent networks onshore, to derive estimates of site response throughout Cascadia (the Pacific Northwest's subduction zone). Results show that the local geologic structure and topography strongly changes shaking from place to place, particularly offshore. The broad-scale, systematic nature of these changes should be considered in assessments of the vulnerability of submarine slopes to future shaking. Results also highlight the need to consider site response more carefully when interpreting the turbidite record to derive chronologies of past earthquakes.